Boundaries

This ecoregion includes the basins of the Manambolo, Tsiribihina, Mangoky, and Onilahy rivers in western Madagascar. 

Freshwater habitats

Western rivers are long, generally slow flowing, and subject to seasonal fluctuations in water level and flow. Tidal influence is significant in the lower reaches of these watercourses (Kiener & Richard-Vindard 1972). Many of western Madagascar’s smaller river basins are dry from April to November (Aldegheri 1972).

Distinctive freshwater habitats within this region include tsingy or karst  formations characterized by fissures, subterranean streams, sinkholes, and caverns produced by erosion, located along the western coast and isolated larger rivers in the central highlands with distinct assemblages of fish species. Lac Itasy, located to the east of the capital Antananarivo in the western central highlands, once supported a diverse assemblage of native fishes, although exotics have almost entirely replaced the native species.

Terrestrial habitats

Deciduous forest is present in the wetter parts of this ecoregion, but drought-tolerant vegetation, such as Didiereaceae and Euphorbia thickets, dominates the landscape (Lowry et al. 1997). 

Justification for delineation

This ecoregion is defined by those basins in western Madagascar with an annual average of less than 1400 mm of rain (Food and Agriculture Organization & International Institute for Applied Systems Analysis 2000). Compared to the wetter northwestern basins, the dry western basins are substantially more influenced by vast seasonal fluctuations in flow rate that could easily wipe out existing fish populations during periods of complete desiccation.

Level of taxonomic exploration

Poor. To date a comprehensive ichthyological survey of the western basins has not been completed and thus comprehensive data are lacking. Many of the western basins are extremely difficult to access, as roads are few and in poor condition.